In printers, copiers, facsimile machines and other imaging devices, it is desirable to be able to track and report how much paper or other media remains in the media tray for use with the device. To this regard, it has been known to include sensing apparatus in the media tray for sensing how full or empty the tray is. Such sensing apparatus may include, for example, a ratcheting mechanism coupled with a media lifting plate in the media tray. As paper is removed from the top of the stack on the media lifting plate for processing in the imaging device, the ratcheting mechanism ratchets the lifting plate up at given intervals, thus keeping the top of the stack of paper available for the media pick mechanism of the imaging device. The incremental ratchet intervals of the ratcheting mechanism are monitored by the imaging device to determine an approximation of how full or empty the tray is with media.
Alternative to a ratcheting mechanism, more sophisticated electronic or light sensors may be employed in connection with the media lifting plate or media itself to determine an approximation of how full or empty the tray is with media. Additionally, in a stationary stack media tray where the pick mechanism is positioned to contact the sheet media, the movement of the pick mechanism may be monitored.
One drawback with conventional media level monitoring mechanisms is that they typically only provide a coarse level of granularity that approximates how full or empty the tray is with media. In other words, only a percentage of how full or empty the tray is can be detected. These mechanisms do not detect the quantity of media actually in the tray. Namely, they do not count or detect a count of how many sheets are in the tray. For example, the ratcheting mechanism or sensors typically only detect coarse levels of granularity in the media tray, such as at levels of 0%, 25%, 50%, 75% and 100%, relative to the tray being full or empty. Although more complicated mechanisms may be employed to improve the granularity for enhanced estimation of how full or empty the tray is, the same are more costly and therefore often undesirable or not feasible in low-end imaging devices that are sensitive to cost issues.
Although a coarse granularity measurement may be sufficient for some users when the media tray is relatively full, a finer granularity or more accurate measurement is often desirable as the tray becomes more empty. For example, when the tray is less than 25% full, there is typically more of a valid concern as to whether sufficient media remains in the tray to finish the next print job (as compared to a 75% or 100% full tray). This is especially true for networked or remotely located imaging devices. Thus, as sheet media is consumed in an imaging device, it is often desirable to know how many sheets actually remain in the tray to avoid running out in the middle of a job.
However, conventional low-cost measurement techniques simply do not detect how many sheets actually remain in the tray because the number may vary depending upon the type, and especially thickness, of the media being used. Typically, media thickness is a variable that is difficult to measure.
Accordingly, an object of the present invention is to provide a method and apparatus for determining sheet count in a media holding tray of an imaging device, regardless of the media thickness (so long as all of the media in the tray is of the same thickness).